The present invention relates to a electrode intended for use with an electric discharge machining apparatus. More specifically, the present invention relates to such an electrode which provides a reduced surface roughness and is well adapted for precision machining operations.
Conventionally, for the material of electrodes used for electric discharge machining, materials such as graphite and copper have been employed. With these materials, the product .rho..multidot..theta.m of thermal conductivity (.intg.) and melting point (.theta.m) is generally high, thereby yielding an electrode having a low consumption rate and high machining rate.
FIG. 1 is a schematic diagram illustrating a conventional electric discharge machining apparatus. In this apparatus, machining is carried out in stages of rough, intermediate and finish (precision) machining by changing both the machining conditions and the electrode for each stage. Generally, however, each of the electrodes is made of the same type of material.
In more detail, in FIG. 1, the electric discharge machining apparatus includes a table 1 on which is positioned a workpiece to be machined, an electrode 2 disposed opposite the workpiece, replacement electrodes 2A through 2D used for varying machining operations, a hydraulic cylinder 3 for controlling the position of the electrode 2 relative to the workpiece, a numeric controller 4 for controlling operations of the various elements of the apparatus of FIG. 1, an X-axis motor 5 for moving the table 1 in the direction of the X-axis, a Y-axis motor 6 for moving the table 1 in the direction of the Y-axis, a power supply 7 for supplying discharge energy between the electrode 2 and the workpiece, and an electrode exchanging apparatus 8 for automatically exchanging the various electrodes 2 and 2A through 2D for the different stages of machining, namely, rough, intermediate and finish machining.
In this conventional machining apparatus employing conventional electrodes, machining operations through intermediate machining to produce a surface roughness of about 10 microns can be accomplished readily. However, difficulties are encountered for finish machining, for example, to provide a surface roughness of less than 5 microns over a wide area. In many cases, it would be desirable to provide a finished surface roughness of 1 micron, 0.5 microns, or less. However, doing so with the conventional apparatus and electrode is quite difficult, and in any event, very time consuming. The reasons for this will now be described.
Referring to FIG. 2, it has been discovered that, even if the amount of discharge energy supplied between the electrode 2 and a workpiece 9 is made as small as possible, over time an electric charge will accumulate in a parasitic capacitor 10 formed between the electrode 2 and the workpiece 9. Eventually, a discharge will occur at projections formed on the electrode 2 or workpiece 9 where the electric field gradient is highest. This unwanted discharge causes roughening of the surface of the workpiece 9.
Referring to FIG. 2, waveforms are shown of the current supplied from the power supply 7 and the actual current flowing between the electrode 2 and the workpiece 9 in the case where the parasitic capacitor 10 is charged. As shown by the waveform in the righthand portion of FIG. 2, the peak value of the current applied between the electrode 2 and the workpiece 9 is much higher (in the case where the parasitic capacitor 10 is charged) than the peak value of the current supplied from the power supply 7. This is an additional reason for unwanted surface roughening of the workpiece 9.
To overcome these problems, it has been proposed, as illustrated in FIG. 3, to divide the electrode 2 into segments and to connect a resistor in series with each of these segments. In the case of FIG. 3, a single switching device is provided in the power supply 7, while in the case illustrated in FIG. 4, separate switching devices 14 are connected in series with each of the resistors 13. In both cases, the capacitance between each segment of the electrode 2 and the workpiece 9 is reduced compared with a single-integral electrode in that the surface area of each of the segments opposed to the workpiece 9 is reduced. This approach, however, is also accompanied by drawbacks. Specifically, some roughening occurs due to the segmentation of the electrode, specifically, due to the presence of boundaries between the segments of the electrodes. Also, in the case of a metal or graphite electrode (the most common conventional electrode materials), if discharge is started between only a single one of the electrode segments and the workpiece, it is difficult to spread the discharge over all of the segments. Accordingly, even though finish machining may be carried out over a number of hours, is very difficult to achieve a finished surface roughness of about 1 to 5 microns.
This will be explained in more detail with reference to FIGS. 5 and 6. As illustrated schematically in FIG. 5, it is assumed that the electrode is divided into two segments 2 and 2'. Current is supplied to the segments 2 and 2' via respective current transformers 15 and 15'. If the discharge first starts between the electrode segment 2 and the workpiece 9, after machining for, for example, ten minutes, due to consumption of the electrode 2, the discharge will shift to the segment 2'. Then, discharge continues between the electrode segment 2' and the workpiece 9 until the electrode segment 2' has been consumed more than the electrode segment 2, at which time the discharge shifts back to between the electrode segment 2 and the workpiece 9. This switching back and forth of the discharge makes it difficult to achieve precision machining.
Furthermore, it may be considered to employ silicon as the material of the electrode. In this case, the concentration of the discharge on only one of the electrode segments 2 and 2' for long periods is prevented, that is, the discharge will be effected between the electrode segments 2 and 2' on the one hand and the workpiece 9 on the other continuously. It has also been considered to use a segmented silicon electrode wherein the electrode is composed of a number of thin silicon plates.
The use of silicon electrodes does allow a finished surface roughness of as small as 1 micron to be obtained over an area of about 20 to 50 cm.sup.2 after a machining time of only several tens of minutes. This is, of course, much shorter than the time required with conventional copper electrodes. It is invented that silicon or other materials which has the same characteristics like silicon will be used as an electrode for EDM.